In the following discussion a III-N semiconductor is a semiconductor having a Group III element and nitrogen. III-N semiconductors such as GaN are useful in fabricating light emitting elements that emit in the blue and violet regions of the optical spectrum. These elements include light emitting diodes and laser diodes. Laser diodes that use semiconductor material based on GaN that emit in the blue and violet regions of the spectrum hold the promise of substantially improving the amount of information that can be stored on an optical disk. However, higher efficiencies are needed for both semiconductor light emitters and photodetectors. This is a particularly urgent problem in GaN-based optical semiconductor devices using BN, AlN, GaN, or InN, which are compounds of nitrogen and Group III elements such as B, Al, Ga, and In and their mixed crystal semiconductors (hereinafter, called GaN-based semiconductors).
Light emitting elements based on III-N semiconductors are typically fabricated by creating a p-n diode structure having a light generating region between the p-type and n-type layers. The diode is constructed from layers of III-N semiconducting materials. After the appropriate layers are grown, electrodes are formed on the p-type and n-type layers to provide the electrical connections for driving the light-emitting element.
One class of blue and green light-emitting diodes (LEDs) or short-wavelength laser diodes (LDs) use GaInN/GaN strained quantum wells or GaInN/GaInN strained quantum wells located between the n-type and p-type layers to generate light by the recombination of electrons and holes injected from these layers. In prior art devices, a strained GaN-based semiconductor layer is constructed by growing a {0001} plane of a GaN-based crystal. The resulting layer has a large piezoelectric field. For example, in a Ga0.9In0.1N strained layer, an extremely large piezoelectric field of around 1 MV/cm is generated.
In addition, III-Nitride semiconductors having a wurtzite crystal structure exhibit a spontaneous polarization. This spontaneous polarization results in sheets of fixed charge at interfaces between III-Nitride layers of different alloy compositions, such as at the interfaces between a III-Nitride quantum well layer and adjacent III-Nitride layers. These charge sheets produce an electric field in the quantum well layer. This electric field, which may also be extremely large, will be referred to herein as a spontaneous electric field.
Usually, when an electric field exists in a quantum well, the energy band of the quantum well layer tends to tilt substantially as the electric field increases. As a result, the wave functions of the electrons and holes separate from one another, and the overlap integrals of both wave functions decrease. Since the optical properties such as the light emission and absorption efficiencies depend on these overlap integrals, the efficiency of these devices decreases with increasing electric fields.
What is needed is a III-Nitride light emitting device in which the problems associated with the internal piezoelectric and spontaneous electric fields have been overcome.